Electro-optically active organic diodes are for example used as organic light emitting diodes (OLEDs), in lightning devices, in display devices and in organic solar cell devices. An organic diode in an organic solar cell device is arranged to generate electricity from light, whilst in a lightning device, the organic diode is arranged to generate light from electricity. Nevertheless, these are just different manifestations of common fundamental properties pertaining to certain electro-optically active organic materials. Progress and development in one area, such as in the area of lightning devices and OLEDs, thus can be utilized for improvements in the other area, such as in the area of organic solar cell devices.
This far, efforts have mostly been spent on devices for generation of light, and in particular on OLEDs. This is partly owing to that the obtainable efficiency, reliability and operational lifetime so far have been considered too poor for viable organic solar cell devices, especially in light of what can be achieved in conventional solar cell devices. Although these properties are desirable to improve also in the area of lightning application devices, the requirements are often not fully as high, and there are already commercially available products, such as displays based on OLED technology. Partly, this is owing to that OLEDs emit light and thus do not require backlighting such as in conventional liquid crystal displays (LCDs). Some other advantages of electro-optically active organic diodes in general are for example that they are comparatively easy and cost efficient to make, can be fabricated in thin, flexible layers and even be made transparent.
Recent electro-optically active organic diodes have shown great progress, especially in efficiency and lifetime, however the increased lifetime has in some cases lead to problems with reliability. Reliability is a problem in general in the area of electro-optically active organic diodes and there is a general need for improved operational life times. Although many of the desired and beneficial properties are owing to the organic nature, there are also some drawbacks that follow from this, for example, organic materials are in general more sensitive to harsh physical treatments and high temperatures compared to many inorganic materials.
One known reason for reliability problems is a reactive cathode. In order to be electron injecting and efficient, low work function electrode materials are typically desired. These, however, are by nature reactive. As a result, cathode materials having a higher work function, but worse efficiency, have been used instead, for example aluminum (Al), which is environmentally stable and also has the advantage of being a known and well explored conducting material from the conventional semiconductor industry. In order to improve efficiency, such higher work function cathode layers sometimes are furnished with an additional thin inorganic injection layer, for example of lithium fluoride (LiF) arranged between the cathode layer and the electro-optically active organic layer.
U.S. Pat. No. 6,525,466 presents an OLED that claims to be reliable and that not to require an injection layer. The OLED consists of a cathode of a mixture, alloy or composite, of a metal and an insulator, for example Al and LiF, which is in contact with the electro-optically active organic layer. For environmental protection there is an optional, conducting capping layer of Al on top of the cathode, i.e. so that the cathode is located between the electro-optically active organic layer and the capping layer.
In conclusion, in the prior art, there exist and are proposed a number of ways to improve reliability and enhance life time in electro-optically active organic diodes, however, there is still a need for further improvements.